A particular type of device in an emitter-switching configuration is constituted by a high-voltage bipolar power transistor and by a low-voltage signal transistor (bipolar or field-effect) in which the emitter terminal of the high-voltage transistor is connected to the charge-collection (collector or drain) terminal of the low-voltage transistor. Typically, the high-voltage transistor has a collector-base, open-emitter breakdown voltage (BV.sub.cbo) which may reach 2 kV, whereas the low-voltage transistor has a breakdown voltage below 60V. The rapid quenching of the low-voltage transistor enables the high-voltage transistor to be quenched extremely quickly and this configuration is therefore commonly used in applications in which the high-voltage transistor is required to be able to switch rapidly between its conducting and cut-off states.
An example of a circuit layout comprising a device 100 in an emitter-switching configuration is shown in FIG. 1. The device 100 comprises a high-voltage bipolar transistor Th the emitter terminal of which is connected to the collector terminal of a low-voltage bipolar transistor Tl. The device 100 is connected in the circuit shown in the drawing by means of the collector terminal Ch and the base terminal Bh of the transistor Th, and by the emitter terminal El and the base terminal Bl of the transistor Tl. In particular, the terminals Bh and Bl constitute the control terminals of the device, enabling the connection between two branches of the circuit connected to the terminals El and Ch to be opened and closed alternately. Typically, the terminal El is connected to a reference terminal (earth); the terminal Ch is connected to the first terminal of a load L, the second terminal of which is connected to the positive terminal of a supply +Vcc (the negative terminal of which is connected to the earth terminal). The device 100 also includes a quenching element B constituted, for example, by a Zener diode or by a chain of diodes connected to the base terminal of the transistor Th and to the emitter terminal of the transistor Tl (or directly to the earth terminal).
During the opening of the device 100, the quenching time of the transistor Th is greater than the quenching time of the transistor Tl because of the large accumulation of charge in its base region. After the transistor Tl is quenched, bringing the emitter current of the transistor Th to zero, the collector current of the transistor Th therefore flows through its base and is discharged to earth through the quenching element B. Once the transistor Th has eliminated all of the residual charge in its base, this transistor is quenched and its collector current, and hence the current in the load L, is brought to zero.
In known devices in an emitter-switching configuration, the quenching element is formed, for example, by discrete components suitably connected to the two transistors; however, this solution increases the production cost and the size of the device as a whole.
In the case of a mixed (signal and power) integrated circuit in which both the low-voltage control circuits and the high-voltage power devices are integrated on the same semiconductor chip, a different known solution consists of the formation of the quenching element within an insulated well of the region in which the control circuits are disposed. A disadvantage of this solution is that it requires the formation of a dedicated insulated recess. This involves greater circuit complexity in order to polarize the insulated recess correctly, and an increase in the size of the device as a whole. Moreover, the insulated recess creates parasitic components with the high-voltage transistor, and these tend to lengthen the quenching times of the device, consequently increasing the power dissipated.
Finally, in both solutions, the quenching element is connected to the base terminal of the high-voltage transistor by means of an external connection (for example, a metal wire) or a dedicated metal track. Known structures therefore have an intrinsic resistance in series with the quenching element resulting from these connections and from the base electrode of the high-voltage transistor, increasing power dissipation at the quenching stage.